1374 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



self-sustaining condition has been achieved. The value of voltage at 

 which the discharge becomes self-sustaining is usually referred to as the 

 breakdown voltage. A detailed discussion of current ranges I and II has 

 been presented by Druyvesteyn and Penning.^ 



Because the tube is a good insulator in current range I at low voltage 

 and suddenly becomes a good conductor in range II, we often think of a 

 gas tube as a voltage controlled device and use it as such in switching 

 circuits. Actually, however, in current range II the tube can be con- 

 trolled only by regulating the current. For this reason in the remainder 

 of this paper, the current is considered as the independent variable. 



In current range III the voltage falls rapidly with increasing current. 

 A space charge of positive ions begins to develop close to the cathode. 

 By the beginning of current range IV most of the voltage drop in the 

 tube appears across this space charge layer. This region of nearly constant 

 voltage with increasing current is called the normal glow discharge. The 

 space charge layer immediately in front of the cathode is conamonly 

 called the cathode dark space. Electrons emitted from the cathode as a 

 result of positive ion bombardment and other processes are accelerated 

 through the high field of this cathode dark space and produce an adjoin- 

 ing layer of intense ionization and excitation called the negative glow. 

 In tubes of the type considered here, this negative glow is the most 

 luminous part of the tube. Beyond the negative glow toward the anode 

 is the so-called Faraday dark space in which no new ionization or ex- 

 citations are produced. Electrons from the negative glow can readily 

 flow through this region to the anode because their space charge is almost 

 completely cancelled by positive ions diffusing from the negative glow. 



Over current range IV the cathode current density is nearly constant. 

 This means that the cross-sectional area of the discharge increases in 

 proportion to the current. This is evidenced by the familiar spreading 

 of the negative glow with increasing current until it covers the entire 

 cathode area. 



In current range V the cathode is completely covered with the negative 

 glow and the current density must increase in direct proportion to the 

 total current. This range of currents is called the abnormal glow dis- 

 charge. 



The current-voltage characteristic of a cold cathode for current ranges 

 IV and V may be modified by changing the geometry from a single plane 

 cathode to a hollow cathode.' A hollow cathode is one in which there is 

 an overlapping of the regions of cathode fall and negative glow from two 

 portions of the cathode. This overlapping can occur on the inside of a 

 cylindrical or spherical surface or between two plane cathodes more or 



